In computer tomography recordings, it can arise that the object to be imaged exceeds the field-of-measurement area, as will be described below in relation to FIG. 1, in which the measurement geometry and the measurement object which result in cut-off projection data are represented. A cone-beam phantom can be used as a measurement object.
This measured projection data, called cut-off or truncated projection data, generates artifacts in the reconstructed CT images. A profile section through an axial image plane runs in the shape of a dish. Consequently, the CT values close to the image edge are clearly too high. As a rule, the CT values in the image center are also not reconstructed correctly. Such CT images are therefore scarcely usable for diagnostic purposes.
While the known solution methods explained below are capable of reducing the so-called truncation artifacts, residual artifacts can nonetheless on closer inspection still be seen.
The aim of reducing or of eliminating truncation artifacts has predominantly been tackled in the literature by completing the cut-off data so as to obtain in this manner the projection profile which would be produced if the field-of-measurement area had captured the entire object. The completion is carried out by extrapolating the cut-off data line. The type of extrapolation differentiates between the following solution approaches discussed in the literature:
B. Ohnesorge et al. [1] describe in “Efficient correction for CT image artifacts caused by objects extending outside the scan field of view”, Med. Phys. 27, Vol. 1, pages 39 to 46, 2000, extrapolation by means of antisymmetrical reflection.
In “Reconstruction from Truncated Projections in Cone-Beam CT using Adaptive Detruncation”, Paper #1506, RSNA 2003, by K. Sourbelle et al. [2], a method of correction of exceeding of the field of measurement is described in which the truncated projection data is supplemented as consistently as possible. To do this, an extended field of measurement has first to be defined on which the projections can then be continued. In contrast to similar methods implemented by the manufacturers, extended consistency criteria are applied here. The image reconstruction itself takes place on the extended field of measurement and is designed not to discontinue at the edge of the physical field of measurement.
In “Hybrid Detruncation Algorithm for the Reconstruction of CT Data”, RSNA paper 2005, C. Penβel et al. [3] combined the ADT algorithm from [2] with an iterative method which also uses the data that can be seen after back-projection outside the actual field of measurement.
An extrapolation method that has been thoroughly tried and tested in practice is the method described in “A novel reconstruction algorithm to extend the CT scan field-of-view”, Med. Phys. 31 (9), September 2004, pages 2385 to 2391, by Hsieh et al. [4], by means of which truncation artifacts that occur when the object to be examined extends into areas outside the so-called field of measurement can be suppressed. The projection images which emerge in the process are termed cut-off or truncated. Truncated projection images generate artifacts in the reconstruction of sectional images. In particular, the image values close to the boundary in the sectional images are as a rule too high and those in a central area too low. The sectional images encumbered with truncation artifacts can therefore be used only to a limited extent for diagnostic purposes. In the method it is assumed that the “cut-off” object is imagined to be continued by a circular water cylinder. The height of the cylinder is equal to the height of the detector and the radius and the position of the midpoint of the circle have to be determined from the projection value and the slope of the projection line at the truncation location, i.e. at the location of the last measured value. As a result of the unavoidable noise of the measurement values, the determination of the slope may not be numerically robust and may consequently yield an incorrect value.
The result is the computation of incorrect extrapolation values and, accompanying this, of an incorrect extrapolation distance. Too short an extrapolation distance reduces only imperfectly the dish-shaped profile of a reconstructed axial section. Too large an extrapolation area overcompensates the “dish” and results in a domed profile section. That means that the outer reconstructed areas of the object are in value terms either still excessively high or depressed and in a gray-scale value diagram will appear lighter or darker than the center of the image. A consequence of this is poor HU fidelity at the object edge and—depending on whether over- or under-compensation has occurred—even in the center of the image.
Further such correction methods are known from J. Starman et al. “Extrapolating Truncated Projections Using 0th and 1st Moment Constraints”, RSNA 2004, and B. Scholz, “Verfahren zur Korrektur von Trunkierungsartefakten[Method for correcting truncation artifacts]”, earlier patent application 10 2006 014 629.8.